1. Field of the Invention
The present invention relates to an image forming method that employs ferroelectrics, and more particularly to a method of forming an image by forming a polarization reversion pattern in ferroelectrics in accordance with image information.
The invention also relates to an apparatus for carrying out the image forming method which employs ferroelectrics, and an image formation medium that is employed in the image forming method.
2. Description of the Related Art
Electrophotography (Carlson method) is widely known as one of the image recording methods. The electrophotography is the process of recording an image, by forming an electrostatic latent image in a photosensitive substance, then developing this electrostatic latent image with toner, and transferring the toner image to recording paper.
The photosensitive substance that is employed in this electrophotography has the disadvantage that the time during which electric potential is held since the surface was charged is short and that electric potential will attenuate (dark attenuation). Because of this, it is necessary to quickly perform the write process of forming an electrostatic latent image in the photosensitive substance uniformly charged. It is also necessary to perform the development and transfer of the formed electrostatic latent image quickly.
In view of the aforementioned circumstances, various investigations and experiments have been made with respect to a memory type photosensitive substance. However, there has not yet been reported any practical memory-type photosensitive substance meeting all the following requirements: (1) memory service life is long; (2) memory formation sensitivity is high; (3) a substance that functions as memory does not degrade the sensitivity of the photosensitive substance; and (4) formation and erasion of memory are reversible.
Hence, there have been proposed a wide variety of image forming methods for forming an electrostatic latent image by use of a medium, having a memory function, which is entirely different from the conventional photosensitive substance employed in the Carlson method.
Such an image forming method has been proposed, for example, in SHINGAKU Technical Report EID 98-180, issued by the Academic Society for Electronic Information Communication. A distribution of heat corresponding to image information is applied to ferroelectrics simultaneously with application of an electric field, in order to form a polarization reversion pattern in the ferroelectrics in accordance with the image information. Then, a change in temperature is applied to the ferroelectrics so that surface charges corresponding to the polarization reversion pattern are generated by a pyroelectric effect. With the surface charges, an electrostatic latent image is obtained.
The image forming method described in the aforementioned Report EID 98-180, however, has the problem that reliability in image formation is low, because an organic polymer material being employed as ferroelectrics, more particularly a vinylidene fluoride polymer, is low in thermal durability.
The present invention has been made in view of the problem found in the prior art. Accordingly, it is an important object of the present invention to provide an image forming method, employing ferroelectrics, which is capable of enhancing reliability in image formation.
Another important object of the invention is to provide an image forming apparatus in which reliability in image formation has been enhanced.
Still another important object of the invention is to provide an image formation medium, formed from ferroelectrics whose thermal durability are high, which is capable of making a contribution to a reliability enhancement in image formation.
To achieve the aforementioned objects of the present invention and in accordance with an important aspect of the present invention, there is provided a first image forming method comprising the steps of: forming a polarization reversion pattern in ferroelectrics in accordance with image information; and obtaining an electrostatic latent image by surface charges corresponding to the polarization reversion pattern; wherein an inorganic ferroelectric oxide is employed as the ferroelectrics.
In accordance with another important aspect of the present invention, there is provided a second image forming method comprising the steps of: subjecting ferroelectrics to a distribution of heat corresponding to image information simultaneously with application of an electric field, in order to form a polarization reversion pattern in the ferroelectrics in accordance with the image information; applying a change in temperature to the ferroelectrics so that surface charges corresponding to the polarization reversion pattern are generated by a pyroelectric effect; and obtaining an electrostatic latent image by the surface charges; wherein an inorganic ferroelectric oxide is employed as the ferroelectrics.
It is preferable that the inorganic ferroelectric oxide be a thin film with metal alkoxides as raw materials and also preferable that the inorganic ferroelectric oxide be LiNbxTa1-xO3 (0xe2x89xa6xxe2x89xa61).
To subject the inorganic ferroelectric oxide to the distribution of heat corresponding to image information, as described above, it is preferable to adopt an exposure method of irradiating infrared light carrying image information to the inorganic ferroelectric oxide. In such a case, it is desirable that the inorganic ferroelectric oxide contain a dopant that absorbs the infrared light carrying image information and also desirable that the dopant be composed of at least any one of elements Mg, Ti, Cr, Ni, Cu, Zn, Zr, Nb, Mo, Rh, Ag, In, Sn, Au, and Pb.
In the second image forming method, it is desirable that a photothermal conversion body in the form of a layer be disposed in close proximity or intimate contact with an image formation layer that consists of an inorganic ferroelectric oxide. The photothermal conversion body is used for absorbing the infrared light carrying image information, converting it into heat, and applying the heat to the image formation layer.
A conductive film may be disposed on one surface of the inorganic ferroelectric oxide, and an electric field may be applied across the inorganic ferroelectric oxide through the conductive film. The conductive film may be constructed of a conducting portion formed over the entire surface, or micro conducting portions and non-conducting portions. The micro conducting portions and non-conducting portions may be alternated in predetermined cycles. In the case of employing the inorganic ferroelectric oxide containing a dopant that absorbs infrared light, it is desirable that the conductive film be transparent to infrared light. In the case where the photothermal conversion body is employed, the conductive film may be an opaque conductive film such as metal.
In the image forming method of the present invention, it is desirable to perform application of an electric field by a corona charging method.
In the image forming method of the present invention, as described above, it is desirable to form a polarization reversion pattern in an inorganic ferroelectric oxide in accordance with image information, obtain an electrostatic latent image by surface charges corresponding to the polarization reversion pattern, develop the electrostatic latent image as a toner image, and transfer this toner image to recording paper.
An image forming apparatus according to the present invention performs image formation by the image forming methods of the present invention described above.
An image formation medium according to the present invention is employed in the image forming methods of the present invention described above and has an image formation layer composed of an inorganic ferroelectric oxide.
In a preferred form of the image formation medium, the image formation layer is constructed of an inorganic ferroelectric oxide containing a dopant that absorbs infrared light carrying image information. The dopant is composed of at least any one of elements Mg, Ti, Cr, Ni, Cu, Zn, Zr, Nb, Mo, Rh, Ag, In, Sn, Au, and Pb.
In another preferred form of the image formation medium, a photothermal conversion body in the form of a layer, which absorbs infrared light carrying image information and converts it into heat and applies the heat to the image formation layer, is disposed in close proximity or intimate contact with the image formation layer.
It is preferable that the inorganic ferroelectric oxide constituting the image formation layer of the image formation medium of the present invention be a thin film with metal alkoxides as raw materials and also preferable that the inorganic ferroelectric oxide be LiNbxTa1-xO3 (0xe2x89xa6xxe2x89xa61).
In the image formation medium of the present invention, it is desirable that a conductive film be disposed on one surface of the inorganic ferroelectric oxide. The conductive film may be constructed of a conducting portion formed over the entire Surface, or micro conducting portions and non-conducting portions. The micro conducting portions and non-conducting portions may be alternated in predetermined cycles. In the case of employing the inorganic ferroelectric oxide containing a dopant which absorbs infrared light, it is desirable that the conductive film be transparent to infrared light. In the case where the photothermal conversion body is employed, the conductive film may be an opaque conductive film such as metal.
The steps of the image forming method, employing ferroelectrics, of the present invention will be described with reference to FIG. 1. As illustrated in the figure, an image formation medium 1 is constructed of an inorganic ferroelectric oxide film 2, which becomes an image formation layer, a conductive film 3 formed on the inorganic ferroelectric oxide film 2 (in FIG. 1, on the bottom surface), and a photothermal conversion film 4 formed on the conductive layer 3.
The image formation medium 1 is polarized in one direction prior to image formation by application of an electric field, as illustrated in FIG. 1A. This process will hereinafter be referred to as a unipolar polarization process. Then, light 5 (e.g., infrared light) carrying image information is irradiated toward the inorganic ferroelectric oxide film 2 through the photothermal conversion film 4, as illustrated in FIG. 1B. This light 5 is absorbed in the photothermal conversion film 4 and converted into heat. With the heat, the inorganic ferroelectric oxide film 2 is heated. At the same time, an electric field is applied across the inorganic ferroelectric oxide film 2 without contact by a corona charging head 6. At this time, at only an exposed portion of the inorganic ferroelectric oxide film 2 irradiated with the light 5, a polarization reversion threshold value for the inorganic ferroelectric oxide film 2 is reduced by heating. Therefore, only the exposed portion is reversed in polarization direction by suitably setting a value for the electric field. In this manner, a polarization reversion pattern is formed in the inorganic ferroelectric oxide film 2 in accordance with image information.
Next, if the temperature of the entire inorganic ferroelectric oxide film 2 is varied, surface charges are generated by the pyroelectric effect, as illustrated in FIG. 1C. Since a polarization-reversed portion and a non-reversed portion have charges of opposite polarities, contrast potential develops, so that an electrostatic latent image is formed. Because electric polarization is high in stability, surface charges occur at any time by varying the temperature of the inorganic ferroelectric oxide film 2. In other words, the image formation medium 1 with the inorganic ferroelectric oxide film 2 has a memory function.
Next, if toner 7 is applied to the inorganic ferroelectric oxide film 2, the toner 7 is attached only to one of the charge polarities by electrostatic force, as illustrated in FIG. 1D. In this manner, the above-mentioned electrostatic latent image is developed as a toner image. The toner 7 can be attached to either only polarization-reversed portions or only non-reversed portions, depending on selection of the toner 7. This toner image, as in the Carlson method, etc., is transferred to recording paper with a transfer charger, etc.
As illustrated in FIG. 1E, a movable electrode 8 is brought into intimate contact with the inorganic ferroelectric oxide film 2, and by applying voltage with a dc power source 9, all polarization directions are reset. If voltage is set higher than the case of the application of an electric field shown in FIG. 1B, all polarization directions can be reset and returned to the initial state in FIG. 1A. That is, reversibility of the polarization reversion makes it possible to erase memory contents arbitrarily. If heat is applied to the inorganic ferroelectric oxide film 2 in the reset or initial state, a polarization reversion threshold value can be lowered.
As described above, the image forming method, employing ferroelectrics, of the present invention has a more stable, electrostatic latent image and is writable and capable of exhibiting superiority over on-demand printing, etc., which are required to print a small number of sheets, to have instancy, and to be writable.
The inorganic ferroelectric oxide, which is employed as ferroelectrics in the image forming method of the present invention, is appreciably higher in thermal durability, compared with organic polymer materials such as the aforementioned vinylidene fluoride polymer, etc. Therefore, in the image forming method of the present invention, which employs an inorganic ferroelectric oxide such as this, image formation reliability becomes sufficiently high.
As described previously, if a distribution of heat corresponding to image information is applied to an inorganic ferroelectric oxide simultaneously with application of an electric field, polarization reversion threshold values for only the heated portions of the inorganic ferroelectric oxide are reduced. Hence, if a value for the electric field is suitably set, polarization reversion occurs only at the heated portions of the inorganic ferroelectric oxide, so that a polarization reversion pattern is formed according to image information.
As described above, in the case where the exposure method, for irradiating infrared light carrying image information to an inorganic ferroelectric oxide, is performed in order to subject the inorganic ferroelectric oxide to a distribution of heat corresponding to image information, heat of the infrared light is absorbed satisfactorily in the inorganic ferroelectric oxide, if the inorganic ferroelectric oxide contains a dopant which absorbs the infrared light carrying image information. Consequently, accurate exposure to infrared light is performed according to image information.
As another method of irradiating infrared light to an organic ferroelectric oxide, a photothermal conversion body in the form of a layer may be disposed in close proximity or intimate contact with an image formation layer consisting of an inorganic ferroelectric oxide. The photothermal conversion body is used for absorbing infrared light carrying image information and converting it into heat and applying the heat to the image formation layer. This case is also capable of performing accurate exposure to infrared light in accordance with image information, because the heat converted efficiently from infrared light is absorbed satisfactorily in the inorganic ferroelectric oxide.
For this exposure to infrared light, high-output infrared laser light, etc., can be employed as the exposure light. This renders it possible to obtain an image pattern without a dark-room process.
In addition, if a conductive film is disposed on one surface of the aforementioned inorganic ferroelectric oxide, and an electric field is applied across the inorganic ferroelectric oxide through this conductive film, the electric field can be applied uniformly across the inorganic ferroelectric oxide.
The conductive film, in addition to a film whose conducting portion is formed over the entire surface, may also be a film constructed of micro conducting portions and non-conducting portions. In that case, if the conducting portions and non-conducting portions are alternated in predetermined cycles, a quantity of surface charge at a polarization-reversed portion can be controlled in the organic ferroelectric oxide by the effect of canceling adjacent charges (i.e., electric charge on a portion corresponding to the aforementioned conducting portion and electric charge on a portion corresponding to the non-conducting portion) occurring on the polarization-reversed portion. More specifically, if a ratio of micro conducting portions and non-conducting portions is 1:1, a quantity of surface charge at a polarization-reversed portion will approach zero. If the quantity of surface charge for the inorganic ferroelectric oxide can be controlled in this manner, selection of toners will become wider in the case of developing an electrostatic latent image as a toner image.
FIG. 2 illustrates how the above-mentioned quantity of surface charge is controlled. An image formation medium 1xe2x80x2 shown in FIG. 2 differs from the image formation medium 1 shown in FIG. 1, in that a conductive film 3xe2x80x2, which consists of micro conducting portions and non-conducting portions alternated in predetermined cycles, is formed instead of the conductive layer 3. Note that the steps shown in FIGS. 2A and 2B correspond to the steps shown in FIGS. 1C and 1D, respectively.
If the conductive film 3xe2x80x2 whose ratio of micro conducting portions and non-conducting portions is 1:1 is employed, as illustrated in FIG. 2A, the surface charge on a polarization-reversed portion will approach zero. Consequently, toner 7 can be attached only on non-reversed portions without being attached on the polarization-reversed portion, depending on selection of the toner 7.
In the case of employing an image formation medium containing a dopant which absorbs infrared light, it becomes necessary for the conductive film to be transparent to infrared light. In the case where the aforementioned photothermal conversion body is employed, an opaque conductive film such as metal may be employed.
The image forming method of the present invention is capable of not only developing an electrostatic latent image with toner and transferring it to recording paper, as described above, but also displaying an image on an image display medium which also serves as an image formation medium. Such an image display method will hereinafter be described in detail.
First, the basic mechanism of an image display method, employing ferroelectrics, of the present invention will be described with reference to FIG. 7. Initially, a description will be given of how a monochrome image is displayed. Reference numeral 101 denotes an image display medium employed in the present invention. The image display medium 101 has a ferroelectric thin film 102, and a contrast display body 103 joined to one surface of the ferroelectric thin film 102. The ferroelectric thin film 102 is formed on a transparent electrode 104 by way of example.
The ferroelectric thin film 102 of the image display medium 101 is subjected to the unipolar polarization process and reset prior to image display by application of an electric field, as illustrated in FIG. 7A. Note that arrows in the ferroelectric thin film 102 of FIG. 7 indicate directions of polarization, respectively. Next, light (e.g., infrared light) 105 carrying image information is irradiated toward the ferroelectric thin film 102 through the transparent electrode 104, as illustrated in FIG. 7B. To make the light 105 carry image information, the light 105 is scanned two-dimensionally on the ferroelectric thin film 112, for example, with the intensity modulated. At this time, only portions of the ferroelectric thin film 102 irradiated with the light 105 are reversed in polarization direction by heating.
Note that in the case where a threshold voltage for the polarization reversion is high, the threshold voltage can be lowered by applying bias voltage across the entire ferroelectric thin film 102, or by raising the ferroelectric thin film 102 to high temperature.
If the polarization direction of the ferroelectric thin film 102 reverses, the surface charge occurring on the surface (joined to the contrast display body 103) is reversed in polarity, so that a charge distribution pattern (electrostatic latent image) is formed in accordance with the polarization reversion pattern. As illustrated in FIG. 7C, the contrast display body 103 emits color in accordance with electric charge. In this manner, only portions of the contrast display body 103 irradiated with the light 105 emit color, so the image carried by the light 105 is displayed on the contrast display body 103.
Note that the ferroelectric thin film 102 basically has a polarization of 180 degrees. This image display medium 101 has a memory function, because polarization reversion does not return to its original state unless a high electric field is applied.
As illustrated in FIG. 7D, if one electrode of a dc power supply 106 is connected to the transparent electrode 104, and an electric field is applied across the ferroelectric thin film 102 without contacting the film 102, for example, by a corona head 107 connected with the other electrode of the dc power supply 106, all polarization directions are reset. If the voltage is set higher than the case of the application of the electric field shown in FIG. 7B, all polarization directions can be reset and return to the state shown in FIG. 7A. That is, reversibility of the polarization reversion allows memory contents to be erased arbitrarily, so that rewriting (i.e., display) is possible.
Next, the case of color image display will be described with reference to FIG. 8. An image display medium 111 in this case has a ferroelectric thin film 112, and a contrast display body 113 joined to one surface of the ferroelectric thin film 112. The ferroelectric thin film 112 is formed on a transparent electrode 114 through a photothermal conversion film 116 by way of example.
A photothermal conversion film 116 is made up of three kinds of heating elements 116R, 116G, and 116B disposed regularly. These three kinds of heating elements 116R, 116G, and 116B selectively absorb infrared light of different wavelengths xcex1, xcex2, and xcex3 and converts into heat, respectively. The basic spectral-absorptance characteristics for these heating elements 116R, 116G, and 116B are illustrated in FIG. 10A. The size of the heating elements 116R, 116G, and 116B corresponds to the size of a pixel required of a display image and is, for example, about a few xcexcm square.
The contrast display body 113 is made up of color-emitting materials 113R, 113G, and 113B, which emit red, green, and blue in accordance with electric charge received from the outside. The color-emitting materials 113R, 113G, and 113B are disposed at positions corresponding to the heating elements 116R, 116G, and 116B, respectively. The size of the color-emitting materials 113R, 113G, and 113B is the same as that of the heating elements 116R, 116G, and 116B.
The ferroelectric thin film 112 of the image display medium 111 is subjected to the unipolar polarization process and reset prior to image display by application of an electric field, as illustrated in FIG. 8A. Then, infrared light 115 of wavelength xcex1 carrying red image information is irradiated toward the photothermal conversion film 116 through the transparent electrode 114, as illustrated in FIG. 8B. To cause the infrared light 115 to carry red image information, the infrared light 115 is scanned in two dimensions on the ferroelectric thin film 112, for example, with the intensity modulated. At this time, only the heating element 116R, irradiated with infrared light 115, of the heating elements 116R, 116G, and 116B of the photothermal conversion film 116 absorbs the infrared light 115 and generates heat.
A portion of the ferroelectric thin film 112 in contact with the heating element 116 generating heat is reversed in polarization direction by the received heat. If the polarization direction of the ferroelectric thin film 112 is reversed, the surface charge occurring on the surface reverses in polarity, so that the color-emitting material 113R at a position corresponding to this polarization-reversed portion (i.e., at a position corresponding to the heating element 116R generating heat) emits red.
In parallel with irradiation of the infrared light 115 of wavelength xcex1, the infrared light of wavelength xcex2 carrying green image information is scanned in two dimensions on the ferroelectric thin film 112. As a result, only the heating element 116G, irradiated with the infrared light of wavelength xcex2, of the heating elements 116R, 116G, and 116B of the photothermal conversion film 116 absorbs the infrared light 115 and generates heat. As with the aforementioned case of red, the color-emitting material 113G at a position corresponding to the heating element 116G generating heat emits green.
Similarly, the infrared light 115 of wavelength xcex3 carrying blue image information is scanned in two dimensions on the ferroelectric thin film 112, and only the heating element 116G, irradiated with the infrared light 115 of wavelength xcex3, of the heating elements 116R, 116G, and 116B of the photothermal conversion film 116 absorbs the infrared light 115 and generates heat. As with the aforementioned case, the color-emitting material 113B at a position corresponding to the heating element 116B generating heat emits blue.
In the manner described above, the color emissions of the color-emitting materials 113R, 113G, and 113B arrayed in the contrast display body 113 are controlled based on red image information, green image information, and blue image information, respectively. Consequently, a full color image can be displayed on the contrast display body 113 in accordance with the red, green, and blue image information.
This case, as with the case of monochrome image display, is also capable of erasing a display image by resetting all the polarization directions of the ferroelectric thin film 112.
The pixel unit of an image to be displayed here is determined by the size of the color-emitting materials 113R, 113G, and 113B of the contrast display body 113. Therefore, if the size of these color-emitting materials 113R, 113G, and 113B and the size of the heating elements 116R, 116G, and 116B are made smaller, resolution for a display image will become higher. As described above, the size of the heating elements 116R, 116G, and 116B is about a few xcexcm square, and it has already been confirmed that polarization reversion can also be controlled in ferroelectrics in the unit of such a size. In practice, it is possible to reduce the pixel size down to the beam diameter of laser light, such as infrared light for scanning.
Note that in some cases, such as where the ferrroelectric thin film 112 itself is able to generate heat when irradiated with infrared light by reason of it containing a dopant that absorbs infrared light, or for other reasons, a color image can also be displayed by a structure such as that shown in FIG. 9. A description will hereinafter be made of this structure. Notice in FIG. 9 that the same reference numerals will be applied to the same elements as those in FIG. 8 and that a description thereof will not be given unless particularly necessary (the same applies to the following description).
In an image display medium 111xe2x80x2 shown in FIG. 9, a light transmission film 117 is disposed instead of the photothermal conversion film 16 employed in the structure of FIG. 8. This light transmission film 117 consists of three kinds of micro filters 117R, 117G, and 117B disposed regularly. These three kinds of micro filters 117R, 117G, and 117B selectively transmit infrared light of different wavelengths xcex1, xcex2, and xcex3, respectively. The basic spectral-transmittance characteristics for these micro filters 117R, 117G, and 117B are illustrated in FIG. 10B.
In this construction, the polarization reversion of the ferroelectric thin film 112 is controlled in the unit of the size of the micro filters 117R, 117G, and 117B by infrared light being transmitted through the micro filters 117R, 117G, and 117B. Therefore, this case is also capable of displaying a full color image on the contrast display body 11 by controlling the colors that are emitted by the color-emitting materials 113R, 113G, and 113B of the contrast display body 113.
In the case where the ferroelectric thin film 112 generates no heat by itself, in the construction of FIG. 9 a photothermal conversion film may be interposed between the ferroelectric thin film 112 and the light transmission film 117. In this case, the photothermal conversion film is caused to generate heat in the unit of the size of the micro filters 117R, 117G, and 117B of the light transmission film 117 by the infrared light transmitted through the micro filters 117R, 117G, and 117B. With the generated heat, the polarization direction of the ferroelectric thin film 112 is reversed.
As has been described above, the image display method employing the image forming method of the present invention displays an image by controlling the polarization reversion of ferroelectrics with irradiation of infrared light, etc. Consequently, the image display method is capable of making a display of an image which has a memory function, by employing inexpensive equipment that does not have such an elaborate drive circuit that controls application of voltage in the unit of a pixel.
The inorganic ferroelectric oxide, which is employed as preferred ferroelectrics in the image display method employing the image forming method of the present invention, is remarkably higher in thermal durability, compared with organic polymer materials such as a vinylidene fluoride polymer, etc. Therefore, in the case where such an inorganic ferroelectric oxide is particularly employed in the image display method of the present invention, image display reliability becomes sufficiently high.
In addition, in the case where infrared light carrying image information is irradiated to a ferroelectric oxide in order to subject the ferroelectrics to a distribution of heat corresponding to image information, heat of the infrared light is absorbed satisfactorily in the ferroelectrics, if the ferroelectric contains a dopant which absorbs the infrared light carrying image information. Consequently, an accurate image can be displayed according to image information.
As another method of irradiating infrared light to ferroelectrics, a photothermal conversion body in the form of a layer may be disposed in close proximity or intimate contact with the ferroelectrics. The photothermal conversion body is used for absorbing infrared light carrying image information and converting into heat and applying the heat to the ferroelectrics. This case is also capable of displaying an accurate image in accordance with image information, because the heat converted efficiently from infrared light is absorbed satisfactorily in the ferroelectrics.
In the case where a combination of micro photothermal conversion portions and non-conversion portions, preferably a combination of micro photothermal conversion portions and non-conversion portions alternated in predetermined cycles, is used as the aforementioned photothermal conversion body, a quantity of surface charge at a polarization-reversed portion can be controlled in ferroelectrics by the effect of canceling adjacent charges (i.e., electric charge on a portion corresponding to the aforementioned photothermal conversion portion and electric charge on a portion corresponding to the non-conversion portion) occurring on the polarization-reversed portion. More specifically, if a ratio of micro photothermal conversion portions and non-conversion portions is 1:1, a quantity of surface charge at a polarization-reversed portion will approach zero.
In such a case, for example, the surface charge on a portion of the ferroelectrics irradiated with infrared light can be made to approach zero, and a portion not irradiated with infrared light will be left in a surface charge state remaining reset. Therefore, in the case where electrochromic material is employed as the contrast display body, for instance, contrast can be displayed by developing an electrochromic phenomenon at the latter portion and not developing it at the former portion. Thus, if a photothermal conversion body, consisting of micro photothermal conversion portions and non-conversion portions, is employed, the degree of freedom for selection of contrast display bodies can be enhanced.
In addition, when a conductive film is disposed on one surface of ferroelectrics, and bias voltage is applied across the ferroelectrics through this conductive film, a combination of micro conducting portions and non-conducting portions, preferably a combination of micro conducting portions and non-conducting portions alternated in predetermined cycles, can be be used as the conductive film. This case is also capable of controlling a quantity of surface charge occurring on the ferroelectrics by the effect of canceling adjacent charges occurring on a polarization-reversed portion. More specifically, a threshold voltage for polarization reversal is reduced at a portion of the ferroelectrics corresponding to the conducting portion by the effect of application of bias voltage, so that polarization reversal is facilitated. On the other hand, such a facilitated operation is not obtained at a portion of the ferroelectrics corresponding to the non-conducting portion. Hence, if a conductive film whose ratio of micro conducting portions and non-conducting portions is 1:1 is employed, for example, surface charge is caused to approach zero by application of bias voltage.
In such a case, by irradiating infrared light to ferroelectrics, for example, the irradiated portion is caused to be in a state where surface charge is not zero. Therefore, in the case where electrochromic material is employed as the contrast display body, for instance, contrast can be displayed by developing an electrochromic phenomenon at a portion irradiated with infrared-light and not developing it at a portion non irradiated. Thus, even if a photothermal conversion body, consisting of micro photothermal conversion portions and non-conversion portions, is employed, the degree of freedom for selection of contrast display bodies can be enhanced.